Breakthrough Discovery: Detection of Ammonia Isotopologues in the Atmosphere of a Chilly Brown Dwarf Unveils a Fresh Path to Exoplanets

Prior to this breakthrough, scientists could only measure isotopologues of carbon monoxide, specifically carbon-12 (¹²C) and carbon-13 (¹³C), within exoplanetary atmospheres. However, a recent achievement by a team of researchers has enabled the detection of ammonia isotopologues in the atmosphere of a frigid brown dwarf.

The recent report published in the journal Nature outlines the successful measurement of ammonia in two isotopologues, specifically ¹⁴NH₃ and ¹⁵NH₃. The study involved contributions from astrophysicists Polychronis Patapis and Adrian Glauser, both affiliated with the Department of Physics and the National Centre of Competence in Research (NCCR) PlanetS. Patapis played a significant role as one of the primary authors of the study.

Quest for ammonia

Brown dwarfs occupy a middle ground between stars and planets, sharing significant similarities with massive gas giants, making them an ideal system for studying such planets. In their research, Patapis and his fellow scientists conducted observations on a specific brown dwarf named WISE J1828. Located approximately 32.5 light years away in the constellation Lyra, this celestial object provides valuable insights into the realm of brown dwarfs and their properties.

WISE J1828 is not visible to the naked eye due to its extremely low effective temperature, which stands at a mere 100°C. This frigid temperature is insufficient for hydrogen fusion to generate the intense light needed to reach Earth. In order to detect this ultracold Y-class dwarf star, researchers pointed the mirrors of the James Webb Space Telescope (JWST) toward the constellation Lyra during the past summer.

With the assistance of the Mid-InfraRed Instrument (MIRI), an infrared detector incorporated on the JWST, the researchers managed to uncover the presence of ammonia isotopologues on WISE J1828. Using the Medium Resolution Spectrometer (MRS) of MIRI, they recorded a spectrum of the brown dwarf spanning wavelengths from 4.9 to 27.9 μm. Within this spectrum, alongside ammonia, the scientists also detected the distinctive absorption bands of water and methane molecules. Specifically, ammonia leads to a reduction in the signal received by the detector within the wavelength range spanning from 9 to 13 μm.

The spectroscopic resolution allows for the differentiation of ammonia isotopologues. If the ammonia molecules contain the less common nitrogen isotope ¹⁵N along with three hydrogen atoms instead of the usual ¹⁴N, the resulting spectrum exhibits a characteristic deviation indicating the presence of ¹⁵NH₃.

Introducing an new exoplanetary diagnostic

The measurement of the ammonia isotopologues in the atmosphere of WISE J1828 is particularly intriguing. According to Patapis and the team, the ratio between ¹⁴NH₃ and ¹⁵NH₃ serves as a tracer, enabling the future examination of star and planet formation. This newly discovered tool promises to facilitate the testing of various established mechanisms for the formation of gas giants.

Gas giants like Jupiter and Saturn are not exclusive to our solar system. They hold a significant place in exoplanet research, as they emerge early in the star formation process and have a decisive influence on the development of smaller, less massive planets. The exact mechanism responsible for the formation of these massive gas giants has remained an unanswered question until now.

Researchers have put forward various theories, yet it remains uncertain whether these planets come into existence through nuclear accretion, similar to the formation of most other planets, or if they result from the gravitational collapse within the protoplanetary disk surrounding the parent star.

The isotopologue ratio determined by Patapis and the research team offers valuable insights. On Earth, the ratio stands at 272 atoms of ¹⁴N for every ¹⁵N atom. Their findings reveal that the ¹⁴NH₃-to-¹⁵NH₃ ratio in WISE J1828’s atmosphere is 670, indicating that this brown dwarf has accumulated fewer nitrogen-15 isotopes during its formation compared to Earth and other planets like Jupiter. Remarkably, the abundance of ¹⁵N is even scarcer on WISE J1828 than on all celestial bodies within our solar system.

Variations in the scenarios of planet formation

Isotope fractionation, the phenomenon involving shifts in isotope abundance, remains a subject of incomplete comprehension. However, it’s theorized that impacts from comets play a role in enriching nitrogen-15 levels due to their notably higher ¹⁵N content. These comet impacts are considered fundamental in the formation of celestial bodies within the solar system, and they are thought to have contributed to the development of Earth’s atmosphere, although the extent of this contribution remains uncertain.

The reduced presence of ¹⁵NH₃ in the spectrum of WISE J1828 indicates a departure from the typical process of planet formation, which primarily involves nuclear accretion. Instead, the findings point to a star-like formation through gravitational collapse. This suggests that gravitational instability, of the type observed, probably plays a significant role in the creation of gas giants, particularly those in extensive orbits around their host stars.

The research paper also addresses another crucial aspect: the ratio of ¹⁴NH₃ to ¹⁵NH₃ seems to exhibit substantial variation depending on the distance between a gas giant and its host star. This variation is exemplified by simulations involving the formation of a planet within the ice lines of both ammonia and molecular nitrogen.

Within the realm of astronomy, ice lines signify the closest distances to the central star where temperatures drop sufficiently for specific volatile chemical compounds to solidify. According to Patapis and the research team, an elevated ¹⁴NH₃-to-¹⁵NH₃ ratio could suggest the accretion of icy materials between the ammonia and nitrogen ice lines during planetary formation.

Astronomers now possess an additional instrument for the direct examination of exoplanets. The ammonia signature’s revelation was made possible by the JWST, reaffirming the immense value and extraordinary capabilities of this space telescope.

Resources

1. ^NEWSPAPER ETH Zurich. (2023, November 7). A new trail to exoplanets: Team successfully detects ammonia isotopologues in atmosphere of cold brown dwarf. Phys.org. [Phys.org]
2. ^JOURNAL Barrado, D., Mollière, P., Patapis, P., Min, M., Tremblin, P., Martinez, F., Whiteford, N., Vasist, M., Argyriou, I., Samland, M., Lagage, P., Decin, L., Waters, L. B. F. M., Henning, T., Morales-Calderón, M., Guêdel, M., Vandenbussche, B., Absil, O., Baudoz, P., . . . Wright, G. (2023). ¹⁵NH₃ in the atmosphere of a cool brown dwarf. Nature. [Nature]